The present invention relates in general to heat treating casting alloys based on aluminum and containing silicon, and also to the resulting castings.
Aluminum-based casting alloys comprise various composition families, most of which are suitable for structural hardening by heat treatment. In particular, mention can be made of the aluminum/silicon/magnesium family typically represented by AlSi7% Mg0.3%, AlSi7Mg0.6%, and AlSi10% Mg0.3% type alloys, and the aluminum/silicon/copper/magnesium family typically represented by AlSi(5% to 10%)Cu(2% to 3.5%)Mg(0.2% to 0.3%) type alloys.
All those alloys are widely used for the mass production of automobile components, for example cylinder heads that are subjected to very high stress while in use. In order to maximize the mechanical properties of such alloys, at least in cases of the most severe stresses, it is usual to carry out heat treatment consisting of solution heat treatment and quenching, followed by age hardening for structural hardening.
One drawback of that kind of treatment is that it can make the alloy very difficult to machine, particularly with structurally hardened alloys including little or no copper (typically, at contents of not more than 1%).
In particular, the machining of very fine threads (for example, threads for fastening injectors to diesel engine cylinder heads), long, small-diameter drilling and the deburring of machined surfaces can present problems (for example, burrs that cannot be broken up are difficult to eliminate by brushing).
Document EP-A-1 065 292 describes a method of cooling a workpiece made of light alloy following solution heat treatment of the workpiece, which can be thought of as staged quenching, by immersing the workpiece in a bath of salt so as to bring its temperature rapidly to a value lying in the range 350° C. to 450° C. Such a known method has the effect of increasing the high-temperature tensile strength of the material of the workpiece, but does not in any way resolve the problems relating to machinability. Moreover, that method results in weaker characteristics at room temperature, which are unacceptable for applications of the combustion engine cylinder head type.
Thus, to date, there is no technique for facilitating the machining of parts made of an alloy of the type with structural hardening or the like.